This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI using steady-state free precession (SSFP) with selective spectral suppression.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MRI process, each nuclear spin responds to four different effects—precession about the main magnetic field, mutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI processes, a combination of these effects occurs periodically.
Fully-reinforced steady-state free precession (SSFP) imaging (also known as balanced SSFP, FIESTA, TreuFISP), provides high signal-to-noise ratio (SNR) efficiency. However, the T2/T1 dependence of the SSFP signal causes fat tissue to appear bright in the reconstructed images. The tissues of interest usually have a comparable or smaller balanced SSFP signal. Therefore, fat-water separation or fat suppression methods have commonly been coupled with SSFP imaging to improve depiction of the structure of interest.
A number of interesting strategies have been devised for reducing or suppressing the fat signal. A simple and effective strategy is to use periodically repeated spectral saturation during the course of acquisition (fat-saturated SSFP); however, transient signal oscillators due to the disruption of the steady-state may lead to artifacts. Another fat-suppression method exploiting the transient signal is fat-saturated TIDE. It is difficult to do 3D imaging and images can be blurred due to overweighting of the central part of k-space. The phase difference due to the chemical-shift between fat and water can be used to separate the two components. Phase-sensitive SSFP is a fast and efficient method requiring only a single acquisition; however, it suffers from partial volume effects. Several other useful multiple-acquisition Dixon-based methods have been proposed for fat-water separation. For these techniques, partial volume effects lead to estimation errors.
A variety of SSFP fat suppression methods reduce the fat signal by creating a stop-band around the fat-resonance. Several proposed spectrally selective fat-suppression methods include fluctuating equilibrium magnetic resonance (FEMR), linear combination SSFP (LCSSFP), periodic flip angle variations and binomial excitation patterns to suppress fat, in- and out-of-phase SSFP imaging and fat suppression alternating repetition time (FS-ATR) SSFT have bene tried. High RF linearity is required for methods varying the flip angle or comprising binomial excitation for suppressing fat.
LCSSFP uses two separate phase-cycled acquisitions and combines them to yield a spectral stop-band around the fat-resonance. The width of the stop-band and the separation between the pass- and the stop-band is determined by the repetition time (TR). On the other hand, FS-ATR uses two different repetition times consecutively played, to create a broad stop-band by aligning the spins precessing at the fat-resonance back to the longitudinal axis. FEMR and LCSSFP put stringent limitations on the possible repetition times, whereas FS-ATR allows for a broader range of repetition times (TR).
A drawback of multiple-acquisition spectrally selective methods, like LCSSFP, is the wedge-shape of the stop-band. The two profiles subtracted from each other are not identical and the SSFP profile is inhomogeneous itself. Imperfect cancellation of SSFP profiles pertaining to different acquisition results in remnant stop-band signal. Therefore, the level of fat-suppression is limited by moderate off-resonant frequency variations.
U.S. Pat. No. 6,307,368 by Vasanwala et al., issued Oct. 23, 2001, entitled “Linear Combination Steady-State Free Precession MRI,” which is incorporated by reference for all purposes describes a steady-state free precession MRI process. U.S. Pat. No. 6,608,479 by Dixon et al., issued Aug. 19, 2003, entitled, “Method and System for MRI with Lipid Suppression,” which is incorporated by reference for all purposes describes another stead-state free precession process.